A variety of power storage units such as lithium-ion secondary batteries, lithium-ion capacitors, and air batteries have been actively developed. In particular, demand for lithium-ion secondary batteries with high output and high energy density (see Patent Document 1, for example) has rapidly grown with the development of the semiconductor industry. Power storage units which can be charged and discharged are essential as power supply sources for various electronic devices, e.g., portable information terminals such as cellular phones, smartphones, and laptop personal computers, portable music players, digital cameras, and medical equipment, for today's information society.
Furthermore, flexible display devices used while being mounted on human bodies or curved surfaces, such as display devices mounted on heads, have been proposed in recent years. This has increased demand for flexible power storage units that can be mounted on curved surfaces. A power storage unit which can be curved or bent is described in Patent Document 2.